1. Field of the Invention
This invention relates to a heat treatment apparatus, and more particularly to a heat treatment apparatus which can uniformly supply a reaction gas to objects. This invention also relates to a method of forming thin films having a uniform thickness on objects by using a heat treatment apparatus.
2. Description of the Related Art
Recent years, heat treatment apparatuses have been used in a film forming step and a thermal diffusion step for manufacturing a semiconductor device. In a heat treatment apparatus, a plurality of semiconductor wafers are stacked horizontally at regular intervals and arranged vertically on a wafer boat, and processed in batches. With this heat treatment apparatus, a semiconductor wafer can be easily disposed, the space and the energy can be saved, and a semiconductor wafers having a large diameter can be processed easily.
When thin films such as an Si epitaxial growth film are formed on a plurality of wafers disposed on a wafer boat, it is required that a thin film formed on a semiconductor wafer have a uniform thickness and the film on each wafer has the same thickness. For this purpose, it is necessary to supply a reaction gas uniformly to the surface of the semiconductor wafers.
FIG. 1 shows a conventional vertical heat treatment apparatus, which has an improved structure for supplying a reaction gas to the wafers so that the film quality can be uniform (see Semicon NEWS, May, 1989, pages 34 to 40). The vertical heat treatment apparatus comprises a reaction furnace having a double tube made up of an outer tube 11 and an inner tube 10 included therein, and a heater 12 arranged around the double tube. The reaction furnace is substantially vertically arranged and surrounded by a heat insulating member 13. A wafer board 15 is arranged in the inner tube 10. A plurality of semiconductor wafers 14 are horizontally supported on the wafer board 15 at regular intervals. The wafer boat 15 is mounted on a turntable 16 connected to a rotational driving means which is provided outside the reaction furnace, and can be moved up and down together with the rotational driving means by a conveying means (not shown).
The inner tube 10 includes a first gas supply tube 17 having a double-tube structure, in which an inner gas supply tube 17b is inserted in an outer gas supply tube 17a. A second gas supply tube 18 is arranged between the inner and outer tubes 10 and 11. The inner gas supply tube 17b has a number of gas supplying nozzles 19 at an end portion thereof in a region corresponding to the length of the wafer boat 15, and communicates with the inner tube 10 at the end portion. The other end of the inner gas supply tube 17b communicates with a gas supply source provided outside the reaction furnace. An end of the outer gas supply tube 17a communicates with the second gas supply tube 18 through the wall of the inner tube 10. The other end of the outer gas supply tube 17a communicates with the gas exhaust source. The other end of the second gas supply tube 18 communicates with a gas supply tube provided outside the reaction furnace. In general, a reaction gas is allowed to flow through the inner gas supply tube 17b, and a coolant gas introduced from the second gas supply tube 18 is allowed to flow through the outer gas supply tube 17b. The coolant gas prevents a reaction product generated by heating the reaction gas from adhering to the inner gas supply tube 17b and the gas supplying nozzles 19, when gas of a material having a low heat-decomposition point is used as a reaction gas.
An exhaust tube 20 is arranged on the opposite side of the first and second gas supply tube 17 and 18. The exhaust tube 20 has a number of gas exhaust ports 21 at an end portion thereof in a region corresponding to the length of the wafer boat 15, and communicates with the inner tube 10 at the portion. The other end of the exhaust tube 20 communicates with an exhaust means provided outside the reaction furnace.
In the above-mentioned heat treatment apparatus, Si epitaxial growth films are formed on the wafers 14 as follows. First, the reaction furnace is heated by heater 12 to a processing temperature, 800.degree. to 1100.degree. C. Then, a reaction gas, SiH.sub.2 Cl.sub.2 gas is introduced from the inner gas supply tube 17b via the gas supplying nozzles 19 to the inner tube 10. At the same time, a coolant gas, or H.sub.2 gas, is introduced from the second gas supply tube 18 to cool the inner gas supply tube 17b, and exhausted from the outer gas supply tube 17a.
The SiH.sub.2 Cl.sub.2 gas introduced in the inner tube 10 is allowed to flow among wafers 14 along the major surface of the wafers in one direction, and is finally exhausted through the exhaust ports 21 outside the furnace. Thus, Si epitaxial growth films are formed on the wafers 14.
However, the above heat treatment apparatus has the following drawbacks. First, since the gas supply tube 17 is arranged in a high temperature region of the inner tube 10 of the reaction furnace, a coolant gas cannot sufficiently cool the reaction gas to prevent deposition of Si, with the result that Si inevitably adheres to the inner gas supply tube 17b and the gas supplying nozzles 19.
Second, in a heat treatment apparatus in general, the lower portion of the reaction furnace has a relatively low temperature, since it is cooled when the wafer boat is loaded into the furnace. Hence, Si may choke the gas supplying nozzles 19 in the upper portion or narrow gas passages thereof. In addition, the amount of Si adhered to the gas supplying nozzles varies depending on the positions of the nozzles. Accordingly, the amount of the reaction gas blown through the gas supplying nozzles varies depending on the positions of the nozzles. In other words, a larger amount of the reaction gas is supplied to the wafers in the upper portion of the reaction furnace, and a smaller amount thereof to the wafers in the lower portion. Therefore, the thin film formed on each wafer in one processing lot has different thicknesses.
Further, when the reaction furnace is heated with Si adhered to the inner gas supply tube 17b, since the material of the inner gas supply tube 17b (quartz) and Si have different heat expansion coefficients, stress is applied to the inner gas supply tube 17b, resulting in breakage thereof.
Furthermore, since the first gas supply tube 17 has a double tube structure, it is difficult to clean it or exchange it with a new one in a case of breakage. Thus, maintenance of the apparatus is very troublesome.
As has been described above, the conventional heat treatment apparatus has various drawbacks and cannot easily form thin films having uniform thickness with respect to each wafer, and improvement thereof has been strongly desired.